The present invention relates to an electronically commutable motor, whose excitation (magnet) windings can be supplied with current via semiconductor output stages from an electronic control unit using pulse-width modulated control signals and, in this context, which generate a rotating (magnetic) field in the stator of the motor, the rotating field placing the permanent magnet of the motor in rotational motion.
In electronically commutable motors, it is necessary to know the position of the permanent magnet rotor with respect to the stator furnished with excitation windings, in order that, given the intended commutation of the excitation windings, the motor, during start-up, is set in rotational motion in the desired rotational direction.
Motors of this type are known that have position sensors, using which the position of the switched-off motor can be made recognizable. These motors require not only supplemental position sensors, but they also require additional outlays in the PWM control unit.
In motors not having position sensors, the position of rotor and stator is detected by evaluating the voltages induced in the excitation windings. However, since when the motor is at rest no voltages are induced that can be evaluated, position detection is not possible when the motor is at rest. Therefore, in starting the determination, a start-up of the motor is not guaranteed, particularly not in the desired rotational direction.
As U.S. Pat. No. 5,327,053 shows, an electronically commutable motor not having position sensors can also be started in the correct rotational direction if, in the start-up phase, a specified position of rotor and stator is first brought about through different currents being supplied to the excitation windings, and the usual flow of current is begun only later. In this context, at the beginning, two excitation windings are fully supplied with current at the same time, and thereafter the current is supplied that is necessary to generate the rotational field having the desired rotational direction.
In this context, however, it has been shown that this driving (activation) in the start-up phase presumes a relatively small mass moment of inertia in the rotor, a high cogging torque, and a powerfully dampened system. This known start-up procedure can therefore not be successfully transferred to motors which do not have these properties.
Therefore, it is an objective of the present invention, in an electronically commutable motor of the type mentioned above having a high mass moment of inertia, a small cogging torque, and poor damping, and not having position sensors, to assure, during start-up, a start in the desired rotational direction.
This objective is achieved according to the present invention due to the fact that, in the start-up of the motor, the control unit, during a specific or specifiable start-up phase, drives the semiconductor output stages in overlapping control phases using PWM control signals, whose pulse-width ratio rises from a minimum to a maximum and then falls again to the minimum, that the overlapping areas of the control phases in the affected excitation windings generate currents which yield a virtually continuous torque curve, and that, in the start-up phase, by shortening the commutation time between successive control phases, the commutation frequency and therefore the rotational speed of the motor is increased.
In the control phases, the excitation windings are supplied with current through the semiconductor output stages having a switching (operating) frequency of the PWM control signals, the pulse-width ratio PWVxe2x80x94the ratio of pulse width to periodxe2x80x94rising from a minimum to a maximum and then falling once again to the minimum. Since the control phases of the excitation windings overlap, the torque is also modulated and, by adjusting the overlapping areas of the control phases of the excitation windings, in the start-up phase, a virtually uniform torque curve can be attained in the desired rotational direction, leading to a reliable start-up in the correct rotational direction. The increase in the rotational speed in the start-up phase is achieved by shortening the commutation time, which occurs continuously in the start-up phase. The commutation time is a function of the motor parameters and is adjusted accordingly.
In accordance with one embodiment, a change in the pulse-width ratio in the control phases can be carried out such that the pulse-width ratio in the control phases increases from the value xe2x80x9c0xe2x80x9d to the value xe2x80x9c1xe2x80x9d and then sinks again to the value xe2x80x9c0,xe2x80x9d the pulse-width ratio being defined as the ratio of pulse width to the period of the PWM control signal, and that the pulse-width ratio, in the start-up phase of the control phases of the PWM control signals, changes over time between the minimum and maximum in a roughly sinusoidal manner.
In accordance with one embodiment, the start-up phase can be defined such that it is selected as a set point time, within which the engine carries out roughly 10 rotations.
According to one embodiment, a quieter and more rapid start-up of the motor is achieved due to the fact that the commutation frequency in the start-up phase increases disproportionately as the time increases.
If, according to a further embodiment, it is provided that the amplitude of the pulse-width ratio in the start-up phase continuously increases and, in continuous running of the motor, gradually changes to a value that is specified through a set point value fed to the control unit, then the motor can be operated in continuous running at varying rotational speeds, the pulse-width ratio PWV taking on the value xe2x80x9c1xe2x80x9d in rated operation at full load.
In order to obtain a uniform torque curve, the adjustment of the overlapping areas of successive control phases is made easier by the fact that the overlapping of control phases that follow one another in time is smaller than one half the period of the control phases.
In the control phases, the PWM control signals are clocked at a switching frequency of, e.g., 20 kHz.